Messages in this thread | | | Date | Thu, 1 Feb 2007 15:52:43 +0100 | From | Jiri Bohac <> | Subject | Re: [patch 0/9] x86_64: reliable TSC-based gettimeofday |
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On Thu, Feb 01, 2007 at 12:20:59PM +0100, Andi Kleen wrote: > On Thursday 01 February 2007 10:59, jbohac@suse.cz wrote: > > > > > Inter-CPU monotonicity can not, however, be guaranteed in a vsyscall, so > > vsyscall is not used by default. > > Only for unsynchronized machines I hope
yes, sorry, only on unsynchronized machines
> The big strategic problem is how to marry your patchkit to John Stultz's > clocksources work which is also competing for merge. Any thoughts on that?
I'll look into that next week. Sorry, I wanted to do that a long time ago, but I spent weeks (over a month) fighting a nasty livelock in the code. (Morale: think twice before using a spinlock inside a {do .. while (read_seqretry(..))} loop)
> >When strict inter-CPU monotonicity is not needed, the vsyscall version of > >gettimeofday may be forced using the "nomonotonic" command line parameter. > >gettimeofday()'s monotonicity is guaranteed on a single CPU even with the very > >fast vsyscall version. Across CPUs, the vsyscall version of gettimeofday is > >not guaranteed to be monotonic, but it should be pretty close. Currently, we > >get errors of tens/hundreds of microseconds. > > I think a better way to do this would be to define a new CLOCK_THREAD_MONOTONOUS > (or better name) timer for clock_gettime().
I absolutely agree. Will do that. This should give userspace a decently accurate and very fast time source.
-- Jiri Bohac <jbohac@suse.cz> SUSE Labs, SUSE CZ
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